The extremely high communications bandwidth of fiber optics technology and transmission systems has revolutionized telecommunications. A single beam of modulated laser light can carry vast amounts of information equaling hundreds of thousands of phone calls or hundreds of video channels. Bandwidth capabilities have been more than doubling every two to three years.
A fiber optic transmission system typically includes the optical transmitter, an optical fiber, an optical amplifier, and an optical receiver.
The optical transmitter receives an electrical digital signal and converts it into an optical is signal by modulating a laser light into optical signal pulses, which represent the various values or states of the electrical digital signal.
The optical signal pulses are transmitted through the optical fiber and, generally, are amplified by one or more optical amplifiers before being converted back into electrical digital signals by the optical receiver. This is generally referred to as the optical link or optical channel.
The optical signal pulses arriving at the optical receiver must be of sufficient quality to allow the optical receiver to clearly distinguish the on-and-off pulses of light signals sent by the optical transmitter. However, noise, attenuation, and dispersion are a few of the impairments that can distort the optical signal pulses, rendering the optical signal pulses marginal or unusable at the optical receiver and making it difficult or impossible to accurately detect or reconstitute the electrical digital signal. The distortion nonuniformly broadens, spreads, or widens the various optical signal pulses, reducing the spacing between the pulses or causing them to overlap, thereby rendering them virtually indistinguishable.
Conventionally, a properly designed optical channel can maintain a Bit Error Rate (“BER”) of 10−13 or better. When an optical channel degrades to a BER of 10−8, a telecommunications system may automatically switch to an alternate optical channel in an attempt to improve the BER. Otherwise, the telecommunications system must operate at a reduced or lowered bandwidth, with poorer overall system performance.
Dispersion is a major contributor to distortion of optical signal pulses, leading to increases in the BER. The distortion caused by dispersion generally increases with increases in the bandwidth or data rate, and with increases in the optical fiber transmission distance.
Dispersion has generally been identified as being caused by (1) chromatic dispersion, or (2) Polarization Mode Dispersion (“PMD”).
Chromatic dispersion occurs when the various frequency components, or colors, of the optical signal pulse travel at different speeds through the optical fiber and arrive at the optical receiver at different times. This occurs because the index of refraction of a material, such as the optical fiber, varies with frequency or wavelength. As a result, the optical signal pulses are distorted through chromatic frequency-related pulse spreading.
Some of the major solutions for chromatic dispersion have included: (1) single-mode propagation, (2) Distributed Feedback (“DFB”) lasers with narrow output spectra, and (3) low attenuation/modified-dispersion optical fibers. All of these advances have contributed to increased bandwidth by allowing the optical signal pulses to pass through the optical fiber with relatively low or reduced dispersion, and hence, relatively low or reduced optical signal distortion.
Single-mode propagation (or use of narrow wavelengths) was achieved through the development of single-mode optical fiber. This optical fiber allows only a single mode of light to propagate through the optical fiber. The DFB laser provides a light source to use with single-mode optical fibers. The DFB laser produces a light with an extremely narrow distribution of output frequencies and wavelengths, minimizing the chromatic dispersion problem. The low attenuation/modified-dispersion optical fiber provides a dispersion-shifted optical fiber that minimizes the speed-vs-wavelength dependency at a specific wavelength.
Previously, chromatic dispersion received greater attention because its adverse effects were initially more limiting at lower available bandwidths and data rates. Now, PMD receives considerable attention due to its potential limitation on optical transparent high-speed long-distance light wave systems, as well as on multi-channel cable television (“CATV”) transmission systems.
PMD refers to distortions in the two orthogonal (right angle) light wave components of the polarized light signal pulses emitted by the optical transmitter. In an ideal optical fiber, which has a perfectly circular cross-section and is free from external stresses, the propagation properties of the two polarized light signal components are identical. However, imperfections introduced in the manufacturing process may result in an optical fiber that is not perfectly circular. In addition, an optical fiber that has been installed may suffer from external stresses such as pinching or bending. These manufacturing imperfections and external stresses cause the two polarization components of the polarized light pulses to have different propagation characteristics, which in turn give rise to PMD.
Despite the manufacturing-induced imperfections, optical fibers (for each optical frequency ω) have two input states (“principal states of polarization”, or “PSP”s) in which a matching light pulse will undergo no PMD spreading. However, light pulses can be input into a fiber in an arbitrary state, and this leads to the pulses being split into two components that propagate independently through the fiber at different velocities. When these components reach the end of the fiber they recombine as two sub-pulses split in time. The delay between the two sub-pulses is designated as the differential group delay (“DGD”), τ.
The DGD and the PSPs of a long fiber are not only dependent on the wavelength or frequency of the optical pulses, but they also fluctuate in time as a result of environmental variations such as temperature changes, external mechanical constraints, and so forth. Their behavior is random, both as a function of wavelength at a given time and as a function of time at a given wavelength.
In a fiber optic transmission system, the optical pulse signal has a bandwidth or range of optical frequencies. “Second order PMD” describes the change of PMD with changing frequency, and is seen as both (i) a changing DGD with the changing optical frequency, and (ii) a changing output polarization with the changing optical frequency.
The impact of first and second order PMD in high bit rate (10 Gb/s) systems has been analyzed. It was found that the second order PMD could lead to important performance losses in addition to the performance penalties caused by the first-order PMD. For the case of large values of chromatic dispersion, second order PMD becomes in fact a major source of performance degradation. Moreover, with the advent of PMD compensators, which typically compensate for the first order effects only (leaving higher orders unaffected or even increasing them), impairments due to accumulated second-order PMD are to be expected.
Second-order PMD is an important issue for a proper assessment of system performance. To emulate the real world fiber, a PMD emulator should not only include the first, but also the second-order. Today's emulators have the strategy to mimic as closely as possible the behavior of long standard fibers with strong polarization mode coupling, both in the time and frequency (wavelength) domain. They typically consist of many segments of high birefringent fibers coupled by rotatable connectors or polarization scramblers. However, the instantaneous PMD (DGD and second-order) value of these PMD emulators is unknown.
Therefore, it is very clear that it is important not only to have controllable first-order DGD, but it is also increasingly necessary to enable methods and apparatus for providing controllable second-order PMD. This is essential for the thorough study, analysis, and testing of real world fiber installations, for a proper assessment of the PMD (including both first order DGD and higher order PMD) induced system penalty, and for the test and analysis of PMD compensators and other optical network components with PMD.
Solutions to problems of this sort have been long sought, but have long eluded those skilled in the art.